1. Field of the Invention
The present invention relates to a method of coding an electronic text document, more particularly, a method of coding an electronic text document to be transferred over a computer network, and apparatus for implementing the method.
2. Description of the Background Art
The recent development of the computer networks enables a rapid and economical distribution of electronic documents to increasingly be available. Especially, with the electronic libraries and the databases, the increasing opportunity at which a huge amount of important and valuable documents are available to the public by database search leads to the problem of illicit distribution of copyrighted material. An exact duplicate of electronic documents may instantaneously be reproduced without involving any troublesome works rather than printed documents.
In order to prevent electronic documents from being illicitly copied, techniques of embedding signature information into electronic documents are proposed. One of the techniques is to slightly shift the distance between the adjacent pairs of lines of words in an electronic document, or line pitch, in dependent upon signature information to be embedded. In accordance with this scheme, the capacity of information to be embedded is dependent upon the number of lines formed by the text on a full page. In adddtion, the line pitch is not uniform through the entire page, which may be noticed during editing.
According to another proposal, the position of a word in a line in an electronic document in a horizontal direction is shifted dependently upon signature information. The signature information conveyed is readable by comparing the document thus coded with its original one. This requires, for decoding, comparison of the coded document with its original. The word processors, e.g. ones for processing European languages, have the function of justifying the rightmost ends of lines. A hard copy of a page, reproduced from an electronic document may often be inconsistent with its original page due to the justification.
The third tactic is of discernibly deforming the font of a specific letter or character on the basis of signature information to be embedded. One of the ways of altering the text formatting or certain characteristics of textual elements is proposed in an article, J. Brassil, et al "Electronic Marking and Identification Techniques to Discourage Document Copying", IEEE Transactions No. 0743-166X/94, pp. 1278-1287, 1994.